Demands for higher data rates for mobile services are steadily increasing. At the same time modern mobile communication systems such as 3rd Generation systems (3G) and 4th Generation systems (4G) provide enhanced technologies, which enable higher spectral efficiencies and allow for higher data rates and cell capacities. Conventional base station transmitter in wireless communication systems like the Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE) system can be split into two separate units, a Remote Unit (RU), which is also referred to as Remote Radio Head (RRH), and a Central Unit (CU), connected either by electrical or optical links through a backhaul network.
One of the interface protocols which enable such a distributed architecture is called Common Public Radio Interface (CPRI). With such architecture, RUs offload Intermediate Frequency (IF) and Radio Frequency (RF) processing from the base station transmitter. Furthermore, the base station transmitter and RF antennas can be physically separated by a considerable distance, thus providing greater deployment flexibility of the system.
The RU comprises a Radio Frequency (RF) front-end, i.e. analog transmit RF components. Typical advanced processing algorithms on RUs include Digital Up-Conversion (DUC), Crest Factor Reduction (CFR) and Digital Pre-Distortion (DPD). DUC interpolates baseband digital signals to a much higher sample rate via a cascade of interpolation filters and further mixes the complex data channels with Intermediate Frequency (IF) carrier signals so that RF modulation can be simplified. Filtering may be needed for baseband digital signal occupying a single component carrier, as is the case in a LTE downlink, in order to get rid of unwanted out-of-band emissions, and comply with regulatory spectrum emission masks (SEM).
One of the reasons for frequency leakage to adjacent channels is that discontinuities often occur between the waveforms of symbols comprised in the baseband digital signal, where the symbols are contiguously arranged. For instance, in the generation of an Orthogonal Frequency Division Multiplexing (OFDM) baseband digital signal, there is generated a discontinuity between an OFDM symbol and the cyclic prefix of the next adjacent OFDM. This signal discontinuity can cause undesired spectral leakage into nearby frequency bands that may need to be suppressed by filtering. Such discontinuity may be suppressed by digital filtering techniques which are usually agnostic to the type of symbols being conveyed by the baseband digital signal.
However, the complexity of the filtering processing may be reduced by considering the type of symbols conveyed by the baseband digital signal during digital filtering.